1. Field of the Invention
The invention relates to an automatic clearance adjuster for a brake or clutch and more specifically to such an adjuster of a cam actuated vehicle brake.
2. Description of the Prior Art
Automatic clearance adjusters employing means to prevent over adjustment of a vehicle brake due to elastic deformation in the brake assembly are well known. Such adjusters are said to be "clearance sensing adjusters" which distinguish between the two parts of any brake stroke, i.e., the clearance or first part of the brake stroke requiring relatively low brake actuating forces for moving the brake shoes to or from the drum and the second part requiring relatively high brake actuating forces for stopping the vehicle. An adjusting mechanism in the adjuster automatically makes a clearance decreasing adjustment when the first part of the stroke exceeds a predetermined amount. The adjusting mechanism is blocked out or prevented from making a clearance decreasing adjustment during the second part of the stroke. The second part of the stroke is often larger than the first part due to the high actuating forces causing deformation in the brake system and therefore can quickly over adjust the brake if the adjusting mechanism is not blocked out. One clearance sensing adjuster, disclosed in U.S. Pat. No. Re. 26,965, employs a disengaging clutch which physically disengages an adjusting mechanism in response to the high brake actuating forces. Another such adjuster, disclosed in U.S. Pat. No. 3,507,369, employs a spring clutch which slips in response to the high brake actuating forces. The clutches employed in these adjusters require a rather high degree of quality control in manufacture and assembly.
Further, the mentioned adjusters provide only for clearance decreasing adjustments in response to the first part of the stroke exceeding a predetermined amount. However, the first part of the brake stroke, i.e., the running clearance, may be temporarily increased when the brake drum thermally expands, whereby the mentioned clearance sensing adjusters will over adjust the brake. An adjuster for automatically increasing and decreasing the clearance of an improperly adjusted brake is disclosed in U.S. Pat. No. 3,990,546. However, this adjuster can not distinguish between the two parts of the brake stroke as do the previously mentioned adjusters employing either the disengaging clutch or the spring clutch.